Relapsing fever is an illness characterized by recurring episodes of fever and nonspecific symptoms (e.g., headache, myalgia, arthralgia, shaking chills, and abdominal symptoms) after infection with one of several species of Borrelia.
In North America, relapsing fever (a zoonosis) is transmitted by the bite of an Ornithodoros tick. In many other parts of the world, including Africa and Asia, relapsing fever is endemic and occurs after the bite of a tick or the human body louse (Pediculus humanus). Tick-borne relapsing fever (TBRF) is also reported from countries in the Middle East, including Israel, Iran, and Jordan. Louse-borne relapsing fever (LBRF) is occasionally imported into the United States by a traveler. TBRF is rarely fatal in North America, where it is most often sporadic; in some African countries (e.g., Senegal and Tanzania), TBRF is a more significant bacterial infection, causing morbidity and death. Conditions that favor infestation with P. humanus, such as living in refugee camps or other stressful situations in which many people are crowded together without access to good hygiene and nutrition, have led to large outbreaks of LBRF with substantial rates of morbidity and death; thus, LBRF is also known as epidemic relapsing fever.
The borreliae are helical or wavy motile spirochetes whose length ranges from 3 to 25 μm and whose width is usually 0.2–0.3 μm. In fixed Wright-stained differential smears, the organisms appear as loose coils (Fig. 172-1). Borreliae are transmitted to humans by exposure to the bite of an infected Ornithodoros tick (TBRF) or to the hemolymph of an infected human body louse, which may be found on clothing (LBRF). For louse-borne disease, it is not the louse's bite that causes transmission; rather, spirochetes are introduced when the louse is crushed (e.g., by scratching) and the insect's infected hemolymph is released and contaminates abraded or normal skin and mucous membranes. Relapsing fever results when variation in borrelial surface antigens leads to repeated bacteremia and stimulation of the immune system by each new antigen. Each time the organism changes its surface antigens, thus evading the immune system, another febrile response occurs. LBRF is caused by Borrelia recurrentis, whereas TBRF is caused by a variety of Borrelia species whose names sometimes correspond to their tick vectors. For example, B. hermsii is transmitted by the tick O. hermsi, and B. turicatae is transmitted by O. turicata (Fig. 172-2).
Photomicrograph of tick-borne relapsing fever spirochete (Borrelia hermsii) in a Wright-Giemsa–stained peripheral blood film. [Reprinted with permission from Dennis DT: Relapsing fever, in Harrison's Principles of Internal Medicine, 17th ed, AF Fauci et al (eds). New York, McGraw-Hill, 2008, p 1054.]
Ornithodoros turicata and O. hermsi, two of the many species of blood-feeding soft ticks responsible for transmitting tick-borne relapsing fever.
(Reprinted from Dworkin et al., 2008, with permission from Elsevier.)
TBRF is endemic in the western United States, southern British Columbia, the plateau regions of Mexico, Central and South America, the Mediterranean, Central Asia, and throughout much of Africa. In the United States, it typically is not reported farther east than Montana, Colorado, New Mexico, and Texas, although cases have been acquired in Oklahoma, Kansas, and Ohio and one case has been reported from Wyoming. A relapsing fever spirochete has been described as far east as Florida, although human infections have not yet been reported in that state. In the United States, exposure sites typically are forested areas at various elevations in mountainous regions (the Cascade, Rocky Mountain, San Bernardino, and Sierra Nevada ranges) and limestone caves in central Texas. Caves may likewise be an important source of TBRF in other areas of the world, such as Israel and Jordan. Houses, cabins, and cowsheds have been implicated as sources of infection because of tick-infested rodent nesting. The most common vector in the United States, O. hermsi, is often found in coniferous forests at elevations of 1500–8000 ft, where it feeds primarily on ground squirrels, tree squirrels, and chipmunks dwelling near freshwater lakes that attract humans who may live in or rent nearby cabins. The disease tends to be most common where humans come into contact with diurnal rodents and their ticks. Only 13 counties have accounted for ~50% of all U.S. cases. Surveillance of TBRF is not performed in all states where the disease is endemic, and substantial underreporting is likely where the disease is reportable. Therefore, the precise distribution of disease is not known.
Many cases of TBRF in West Africa have been attributed to infection with B. crocidurae transmitted by the tick O. sonrai. In Senegal, this disease has been identified as the most common bacterial infection causing febrile illness. Thus this infection is an important factor in the differential diagnosis of suspected malaria in West Africa, where LBRF has not been described in many decades. Co-infection with Plasmodium species was reported in more than one-third of blood films from Senegalese TBRF patients. In eastern sub-Saharan Africa, B. duttonii is more prevalent. TBRF has been detected (albeit less commonly) in northern Africa (e.g., in Morocco), where B. hispanica and B. crocidurae have been identified.
The epidemiology of LBRF is not as well characterized as that of TBRF, probably in part because of the higher prevalence of the former in regions with relatively few resources for communicable disease surveillance. Historically, LBRF has been described in North America and Europe, but it is now only uncommonly reported in these regions. LBRF is relatively well described in East Africa. Outbreaks have been reported in Sudan and Ethiopia. Reports of disease in the highlands of Ethiopia have included many documented cases, despite a recent decline; in that country, more cases have occurred in ...